1. Technical Field
Example embodiments of the present invention relate in general to a technology for wireless telecommunications, and more specifically, to a method for transmitting signals using multiple antennas, which can increase capacity of wireless communication system.
2. Related Art
A technology of using multiple antennas such as Multiple Input and Multiple Output (MIMO) is being used for increasing capacity of wireless communication system. The technology of multiple antennas may be classified into a technology of beamforming to achieve array gain using correlation between antennas and a technology of spatial multiplexing (SM) to achieve multiplexing gain using un-correlation between antennas.
The technology of beamforming may provide targeted receivers with high signal-to-noise ratio (SNR) by forming electromagnetic wave having directivity toward a specific direction, and make other receivers except the targeted receivers not to receive interference.
Thus, a base station of a cellular communication system may use the technology of beamforming so as to provide high quality communication services to terminals located at edge of cell belonging to the base station, and may use a technology of Space Division Multiple Access (SDMA) based on the technology of beamforming so as to provide services to a plurality of terminals through the same frequency spectrum.
On the other hand, a multiplexing gain may be achieved when correlation between multiple antennas is low. According to an information theory, an enhancement of system capacity due to multiplexing gain is usually larger than an enhancement of system capacity which can be achieved by using beamforming.
However, high SNR is required to use the technology of spatial multiplexing. Also, although high number of MIMO channel rank is demanded in order to achieve effect from use of the technology of spatial multiplexing, practically it may not be realized. Generally, high SNR region in a cell is a center region adjacent to the base station of the cell which cannot achieve enough high number of channel rank. Also, regions far from the base station which are beyond line-of-sight (LOS) area may have enough high number of channel rank, but SNR may be low in the regions. Therefore, an environment to which the technology of spatial multiplexing can be applied is restricted.
According to an information theory, an enhancement of system capacity due to multiplexing gain is superior to an enhancement of system capacity due to array gain. However, environments suitable for pursuing multiplexing gain and those for array gain may be separated. Thus, a commercialized wireless communication system pursuing multiplexing gain and a commercialized wireless communication system pursuing array gain may exist separately.